Corrosion of underground water, oil and gas metal pipelines is a serious problem requiring substantial expenditures for protection systems, maintenance and replacement. Dependent upon the pipeline metal and the physical and chemical composition of the soil in which the pipes within the pipeline are located, such corrosion is frequently ascribed to galvanic action in which corrosion current flows from anodic areas on the pipeline through the surrounding soil, which acts as an electrolyte, to cathodic areas on the pipeline. Generally, where the current leaves the anodic areas, corrosion of the pipeline occurs.
In many cases, the exterior of the pipelines are coated with a suitable non-metallic protective coating of an electrically insulating material. The coating isolates the metal of the pipeline from the surrounding electrolyte (i.e. the soil), thereby interposing a high electrical resistance in the anodic-cathodic circuit so that no significant corrosion current flows from the anodic areas to the cathodic areas of the pipeline.
Another prior art corrosion control attempt which is generally used in combination with the pipeline exterior coating is external cathodic protection. In this attempt, direct current is introduced into the soil surrounding the pipeline to oppose the discharge of the corrosion current from anodic areas of the pipeline, whereby the entire exposed exterior surface of the pipeline becomes a single cathodic area.
The required current is provided by impressing a voltage from an outside electrical power system on a circuit between the pipeline and a ground bed in the soil comprising buried electrodes of graphite, carbon, non-corrodable alloys, platinum or the like. Alternatively, the current may be generated by means of galvanic anodes of a less noble or electronegative metal, (e.g. zinc, aluminum or magnesium) buried in the soil and electrically connected to the ferrous metal pipeline which then becomes the cathode of the galvanic couple. In this attempt, the galvanic anode corrodes as current is discharged to the pipeline This method is normally used where only a relatively small current is required for protection.
Although under some conditions external cathodic protection may be employed successfully with a bare or uncoated pipeline, corrosion protection is generally realized by the combined use of the coating system and the cathodic protection system. In such cases, external cathodic protection system serve to protect the exposed steel at a small number of unavoidable defects or holidays in the coating.
A further aspect of a corrosion protection system is a pipeline support. Terrestrial pipeline emplacements in rocky or stony areas are protected by the addition of sand as an initial backfill in and around the pipeline. The introduction of the layer of sand or other suitably granular material, helps to protect the pipeline coating from stone or rock damage while the pipeline trench is backfilled with the excavated, stony material. A layer of sand padding can either be placed into the trench prior to pipeline placement to protect the pipeline from the stony trench bottom or, as is commonly the case, some other form of pipeline support is placed in the trench prior to pipeline placement. In this way the pipeline is safely held off the trench bottom and the sand backfill flows down around the pipeline easily when dumped in. Typically, high density foam cushions are being used for this pipeline support. However, it is known that where a foam support contacts the pipeline surface, the anode to cathode circuit can be compromised and any adjacent holidays in the pipeline coating become likely sites for corrosion damage.
Therefore, there is provided an improved pipeline support which overcomes problems in the prior art.